Xin He

A highly conductive, flexible, transparent composite electrode based on the lamination of silver nanowires and polyvinyl alcohol

In this study, a spin-coating method is demonstrated for producing silver nanowire films on flexible polyethylene terephthalate substrates. To improve the inherent, poor uniformity in conductivity and lower the high surface roughness of silver nanowire films, a composite film based on a silver nanowire network and polyvinyl alcohol was fabricated by a mechanical lamination technique. The effects of mechanical pressure and lamination time on the photoelectric characterization of the composite films were investigated. Stable electrical channels of a silver nanowire network were established on the substrate by laminating the composite layers using a pressure of 30 MPa for 10 min. A transmittance of 86.9% at a wavelength of 550 nm with a sheet resistance of 0.75 Ω sq−1 and a haze of 7.1% were measured for the silver nanowire–polyvinyl alcohol film. Furthermore, the composite film also showed low surface roughness, stable conductivity after 300 bending cycles, as well as the improved weak adhesion of nanowires to the substrate due to the compact contact between silver nanowires and polyvinyl alcohol.

PVP-Assisted Solvothermal Synthesis of High-Yielded Bi2Te3 Hexagonal Nanoplates: Application in Passively Q-Switched Fiber Laser.

High-yielded Bi2Te3 hexagonal nanoplates were fabricated via a facile solvothermal method with the assistance of poly (vinyl pyrrolidone) (PVP). Effects of PVP molecular weight and concentration on the morphology and size distribution of the products were illustrated in this study. Molecular weight of PVP is significant for determining the morphology of Bi2Te3. The hexagonal nanoplates with high yield were obtained in the presence of PVP with molecular weight of 40000–45000. The average size and size distribution of Bi2Te3 nanoplates can be slightly varied by controlling concentration of PVP. High-yielded Bi2Te3 nanoplates exhibit characteristics of saturable absorption, identified by open-aperture Z-scan technique. The synthesized Bi2Te3 nanoplates with large saturation intensity of 4.6 GW/cm2 and high modulation depth of 45.95% generated a stable passively Q-switched fiber laser pulse at 1.5 μm. In comparison with recently reported Q-switched fiber lasers utilizing exfoliated Bi2Te3 nanosheets, our passive Q-switching operations could be conducted at a relatively low threshold power of 30.2 mW or a quite high output power of 99.45 mW by tuning the cavity parameters.

Ultrafast saturable absorption in topological insulator Bi 2 SeTe 2 nanosheets

Topological insulator (TI) Bi2SeTe2 nanosheets with very regular hexagonal morphology were synthesized by a hydrothermal route. Open aperture (OA) z-scan method was performed to measure the saturable absorption (SA) characteristics of the as-prepared TI Bi2SeTe2 nanosheets. The measured modulation depth, saturation intensity and nonsaturable loss of the sample were 61.9%, 4.46 GW/cm2 and 4.5% respectively. An ultrafast intraband scattering time of ~50 fs was obtained through simulating the SA curve, which indicates the TI Bi2SeTe2 nanosheets may be a good candidate for mode-locking material.

Versatile preparation of ultrathin MoS 2 nanosheets with reverse saturable absorption response

High-yielded ultrathin MoS2 nanosheets (UMS) with thickness below 4 nm were successfully synthesized by a simple, cost-effective and reproducible solid-state reaction method. Significant reverse saturable absorption and nonlinear refraction responses of the UMS were measured by the z-scan experiment under femtosecond pulses at 800 nm. The figure of merit is calculated to be ~2.52 × 10−15 esu cm. Furthermore, optical limiting (OL) effects of the UMS were observed with low threshold FOL ~44 mJ/cm2. These results reveal that solid-state reaction is a feasible method for the fabrication of optical nanomaterials used in nanophotonic devices including optical limiter, which can be expanded to prepare other two-dimensional nanomaterials.

Ultra-broadband nonlinear saturable absorption of high-yield MoS2 nanosheets

High-yield MoS2 nanosheets with strong nonlinear optical (NLO) responses in a broad near-infrared range were synthesized by a facile hydrothermal method. The observation of saturable absorption, which was excited by the light with photon energy smaller than the gap energy of MoS2, can be attributed to the enhancement of the hybridization between the Mo d-orbital and S p-orbital by the oxygen incorporation into MoS2. High-yield MoS2 nanosheets with high modulation depth and large saturable intensity generated a stable, passively Q-switched fiber laser pulse at 1.56 μm. The high output power of 1.08 mW can be attained under a very low pump power of 30.87 mW. Compared to recently reported passively Q-switched fiber lasers utilizing exfoliated MoS2 nanosheets, the efficiency of the laser for our passive Q-switching operation is larger and reaches 3.50%. This research may extend the understanding on the NLO properties of MoS2 and indicate the feasibility of the high-yield MoS2 nanosheets to passively Q-switched fiber laser effectively at low pump strengths.

Screen-Printed Fabrication of PEDOT:PSS/Silver Nanowire Composite Films for Transparent Heaters

A transparent and flexible film heater was fabricated; based on a hybrid structure of poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) and silver nanowires (Ag NWs) using a screen printing; which is a scalable production technology. The resulting film integrates the advantages of the two conductive materials; easy film-forming and strong adhesion to the substrate of the polymer PEDOT:PSS; and high conductivity of the Ag NWs. The fabricated composite films with different NW densities exhibited the transmittance within the range from 82.3% to 74.1% at 550 nm. By applying 40 V potential on the films; a stable temperature from 49 °C to 99 °C was generated within 30 s to 50 s. However; the surface temperature of the pristine PEDOT:PSS film did not increase compared to the room temperature. The composite film with the transmittance of 74.1% could be heated to the temperatures from 41 °C to 99 °C at the driven voltages from 15 V to 40 V; indicating that the film heater exhibited uniform heating and rapid thermal response. Therefore; the PEDOT:PSS/Ag NW composite film is a promising candidate for the application of the transparent and large-scale film heaters.

Temperature-controlled transparent-film heater based on silver nanowire-PMMA composite film.

We fabricated a high-performance film heater based on a silver nanowire and polymethyl methacrylate (Ag NW-PMMA) composite film, which was synthesized with the assistance of mechanical lamination and an in situ transfer method. The films exhibit excellent conductivity, high figure of merit, and strong adhesion of percolation network to substrate. By controlling NW density, we prepared the films with a transmittance of 44.9-85.0% at 550 nm and a sheet resistance of 0.13-1.40 Ω sq-1. A stable temperature ranging from 130 °C-40 °C was generated at 3.0 V within 10-30 s, indicating that the resulting film heaters show a rapid thermal response, low driving voltage and stable temperature recoverability. Furthermore, we demonstrated the applications of the film heater in defrosting and a physical therapeutic instrument. A fast defrosting on the composite film with a transmittance of 88% was observed by applying a 9 V driving voltage for 20 s. Meanwhile, we developed a physical therapeutic instrument with two modes of thermotherapy and electronic-pulse massage by using the composite films as two electrodes, greatly decreasing the weight and power consumption compared to a traditional instrument. Therefore, Ag NW-PMMA film can be a promising candidate for diversified heating applications.

Vertically standing layered MoS 2 nanosheets on TiO 2 nanofibers for enhanced nonlinear optical property

Vertical layered MoS₂ nanosheets rooting into TiO₂ nanofibers were successfully prepared by a facile two-step method: prefabrication of porous TiO₂ nanofibers based on an electrospinning technique, and assembly of MoS₂ ultrathin nanosheets through a simple hydrothermal reaction. Significant enhancement of nonlinear optical response of the MoS₂/TiO₂ nanocomposite was confirmed by an open-aperture z-scan measurement. The nanocomposite displayed strong optical limiting (OL) effects to ultrafast laser pulses with a low OL threshold of ~22.3 mJ/cm², which is lower than that of pristine TiO₂ nanofibers and MoS₂ nanosheets. In addition to the contribution of the strong nonlinear absorption of MoS₂ nanosheets and TiO₂ nanofibers, such phenomenon is also attributed to the unique structure of vertically standing layered MoS₂ nanosheets on TiO₂ nanofibers with a large amount of exposed edge states, large surface areas and fast electron transfer between TiO₂ and MoS₂. This work broadens our vision to engineering novel hierarchical MoS₂-based nanocomposite for efficiently enhanced nonlinear light-matter interaction.

Flexible and transparent optically anisotropic films based on oriented assembly of nanofibers

Nanofibers are characterized with unique electronic, magnetic and optical properties, due to their extremely high aspect ratio and large specific surface area. Assembly of nanofibers with predesigned macro architectures is a key step toward practical applications. Herein, we demonstrate the scalable oriented assembly of nanofibers based on electrospinning, and explore their potential application in constructing optically anisotropic films. By the post soaking-and-drying approach, the nonwoven films assembled by oriented polymer nanofibers can be readily converted to flexible films with high transparency, which show optical transmission contrast exceeding 0.9 in the visible to near infrared region. The method for preparation of optically anisotropic films proposed here circumvents the sophisticated dye-doping and thermal drawing processes which are inevitable in conventional approaches. Considering the scalability potential of the electrospinning technique, the method demonstrated here is promising for industrial production of film polarizers.

Study on the Metal-Enhanced Fluorescence of Dyes by Ag-Polyvinylpyrrolidone Nanocomposites

In this work, the effects of Ag nanoparticles or Ag-PVP nanocomposites on the fluorescence properties of rhodamine B and fluorescein were investigated. The fluorescence intensities of the dyes could be largely enhanced by Ag nanoparticles with various concentrations. Moreover, the intensities were further increased by Ag-PVP nanocomposites. The results show that the maximum enhancement ratio of 36.5 fold is achieved for rhodamine B in the presence of 2% Ag-PVP nanocomposites, as well as 4.37 fold for fluorescein with 5% Ag-PVP nanocomposites. The enhancement is believed to mainly be originated from the increased excitation rate of the dyes due to the local electromagnetic field which is improved by the interaction of light with Ag nanoparticles. The further fluorescence enhancement of the dyes by Ag-PVP nanocomposites is attributed to the effective separation of Ag nanoparticles from the dyes by PVP molecules. Additionally, the difference in the enhancement ratio of two dyes was also discussed. The lower quantum yield of the dye, as well as higher overlapped degree between Ag nanoparticles and the absorption of dye result in the larger enhancement ratio. The research could be meaningful for improving the sensitivity of rapidly medical or biological assays.